1. Field of the Invention
The present invention relates to an AC generator mounted in a vehicle.
2. Description of the Related Art
Reduction in fuel consumption of vehicles has been required by various regulations including CAFE corresponding to the earth environmental protection. Accordingly, the size and weight of AC generators have been reduced by reducing various losses of the AC generator, thereby improving the efficiency of the generator.
Main losses of the AC generator are copper loss of the stator coil and field coil, rectifier loss generated during the conversion from AC power to DC power, core loss including eddy current loss and hysteresis loss, and the mechanical losses including cooling fan loss and brush loss. The copper loss, rectifier loss, core loss and brush loss can be reduced by traditional ways of selection of material to be used or shapes to be designed. However, it is difficult to improve cooling efficiency of the generator.
A rotor disclosed in JP-U-5-11769 is designed to be flat to increase the surface area of field coil in contacted with the pole core, thereby improving heat-dissipation. Although the increased heat-dissipation improves the efficiency of the AC generator, the radial size of the AC generator necessarily increases, resulting in difficulty to provide a radially-wider space for the generator in the vehicle engine compartment.
An AC generator disclosed in JP-B2-59-42546 has an axial flow type cooling-fan to improve the cooling effect. However, it is difficult to increase an amount of cooling air as the size of the generator become smaller.